additions to metadata object api: more vorbiscomment functions, trailing-null on vorbis comment field values enforced everywhere

add extra verbiage about hardware problems to verify error message, hopefully this will cut down on the number of bogus bug reports

add more informative error info for some common errors; also, no not exit with error when decoding to stdout and writing return EPIPE

make --serial-number or the default random serial number apply only to the first stream, and increment it for subsequent streams

print error if output file already exists, instead of overwriting by default; add -f (--force) option to force overwriting of files

when encoding to Ogg FLAC, set a random serial number (instead of 0) if one is not provided on the command-line

use the OggFLAC__FileDecoder instead of the old OggFLAC__StreamDecoder way for all Ogg FLAC decoding, and enable use of --skip with Ogg FLAC

fix up reporting of the current frame to the write callback; also fixes a problem with progress callbacks in the ogg flac file encoder